U.S. patent number 6,632,362 [Application Number 09/766,140] was granted by the patent office on 2003-10-14 for vacuum retort anaerobic digestion (vrad) system and process.
Invention is credited to Herman P. Miller, III.
United States Patent |
6,632,362 |
Miller, III |
October 14, 2003 |
Vacuum retort anaerobic digestion (VRAD) system and process
Abstract
This invention pertains generally to anaerobic digesters, and
more particularly to a system and process that extends the range of
acceptable biomass feedstock concentrations and provides pH process
control of all known digestion processes: by the creation of a
vacuum or vacuum retort in or before the digester that essentially
acts to boil off excess liquid, thickening the liquor of the
digester to an optimum value; and to control the pH of the various
phases or stages of digestion by separating the component products
of digestion and mixing or diffusing a portion of said component
products into the mixing system of each phase of the process. A
system and process: that produces a potable water effluent, clean
commercially useful by-products, zero environmental emissions, a
very positive ecological impact; and, that has the capability of
being completely self powered plus providing energy back to the
community or industry that supports or owns it.
Inventors: |
Miller, III; Herman P.
(Stockton, CA) |
Family
ID: |
25075530 |
Appl.
No.: |
09/766,140 |
Filed: |
January 19, 2001 |
Current U.S.
Class: |
210/603; 210/181;
210/205; 210/258; 210/609; 210/613; 210/631 |
Current CPC
Class: |
C02F
3/28 (20130101); Y02E 50/30 (20130101); C02F
1/66 (20130101); Y02E 50/343 (20130101); C02F
2301/063 (20130101) |
Current International
Class: |
C02F
3/28 (20060101); C02F 1/66 (20060101); C02F
003/28 () |
Field of
Search: |
;210/603,609,612,613,615,616,617,631,175,180,181,205,252,258,259 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Prince; Fred G.
Parent Case Text
RELATED PATENT
The invention described herein makes use of the process and system
of U.S. Pat. No. 6,291,232, the disclosure of which is incorporated
herein by reference.
Claims
What is claimed is:
1. In an anaerobic digestion system: a retort vessel containing a
liquor comprising a liquid and a digestible biomass, means for
maintaining the liquor at a thermophilic temperature, a space above
the liquor in which liquid vapor and gaseous products of digestion
can collect, means including a condenser, a chiller and a pump for
removing the gaseous products of digestion and the liquid vapor
from the vessel and thereby creating a vacuum which lowers the
pressure at the surface of the liquor and causes the liquid to boil
and vaporize at the thermophilic temperature.
2. The system of claim 1 further including means for introducing a
raw sludge influent into the vessel above the liquor, and means for
receiving sludge effluent from the lower portion of the vessel.
3. The system of claim 1 further including means for reducing the
temperature of the condensed liquid vapor to dissolve gaseous
components therein.
4. The system of claim 1 further including a vacuum oven for drying
the sludge effluent into a sludge cake outside the retort
vessel.
5. In an anaerobic digestion system: a retort vessel containing
digestible biomass solids in a liquor, a dome above the liquor
containing a vapor and gaseous products of digestion, means for
condensing the vapor at a distance above the dome equal to
approximately one atmosphere of water column to create a vacuum at
the surface of the liquor which vaporizes the liquor and
concentrates the biomass solids, and means for removing the gaseous
products and the vaporized liquor from the enclosed space.
6. The system of claim 5 wherein the liquor is water.
7. The system of claim 5 including means for heating the liquor
externally of the enclosed space in order to promote concentration
of the biomass solids.
8. The system of claim 5 wherein the gaseous products of digestion
include hydrogen sulfide, carbon dioxide and methane.
9. The system of claim 5 including means for introducing the
biomass solids into the dome at a temperature/pressure gradient
above the vaporization point of the liquor.
10. The system of claim 5 further including means for introducing a
raw sludge influent into the vessel above the liquor, and means for
receiving sludge effluent from the lower portion of the vessel.
11. The system of claim 5 further including means for reducing the
temperature of the condensed vapor to dissolve gaseous components
therein.
12. The system of claim 11 wherein the gaseous components to be
dissolved are selected from the group consisting of hydrogen
sulfide, carbon dioxide, and mixtures thereof.
13. The system of claim 5 including means for dissolving any
hydrogen sulfide in the gaseous components into the liquor, and
means for adding a caustic component to neutralize hydrosulfuric
acid formed by hydrogen and sulfur ions.
14. The system of claim 13 wherein the means for adding a caustic
component includes means for adding sodium hydroxide to form a salt
that precipitates from solution in crystalline form at low
temperatures, and further including means for removing the salt
from the vessel.
15. The system of claim 5 further including means for releasing the
surface tension of the liquor so that methane gas can bubble
off.
16. The system of claim 5 further including means for reheating the
liquor and releasing the surface tension of the liquor to allow
carbon dioxide gas to bubble off.
17. In an anaerobic digestion process, the steps of: introducing a
raw sludge influent into a retort vessel containing a liquor of
digestible biomass solids in a liquid, with a vapor and gaseous
products of digestion collecting in a space above the liquor,
maintaining the liquor at a thermophilic temperature, and removing
the gaseous products of digestion and the liquid vapor from the
vessel in a manner that creates a vacuum at the surface of the
liquor to vaporize the liquid and concentrate the biomass
solids.
18. The process of claim 17 wherein the gaseous products of
digestion and the liquid vapor are pumped from the space above the
liquor, condensed and cooled to create the vacuum at the surface of
the liquor.
19. The process of claim 17 including the step of heating the
liquor externally of the enclosed space in order to promote
concentration of the biomass solids.
20. The process of claim 17 wherein the gaseous products of
digestion include hydrogen sulfide, carbon dioxide and methane.
21. The process of claim 17 including the step of introducing the
biomass solids into the space above the liquor at a
temperature/pressure gradient above the vaporization point of the
liquor.
22. The process of claim 17 wherein the vacuum is produced at the
surface of the liquor by condensing the vapor at a distance above
the surface equal to approximately one atmosphere of water
column.
23. The process of claim 17 further including the step of reducing
the temperature of the condensed vapor to dissolve gaseous
components therein.
24. The process of claim 23 wherein the gaseous components to be
dissolved are selected from the group consisting of hydrogen
sulfide, carbon dioxide, and mixtures thereon.
25. The process of claim 17 including the steps of dissolving any
hydrogen sulfide in the gaseous components into the liquor, and
adding a caustic component to neutralize hydrosulfuric acid formed
by hydrogen and sulfur ions.
26. The process of claim 25 wherein the caustic component is added
by adding sodium hydroxide to form a salt that precipitates from
solution in crystalline form at low temperatures, and removing the
salt from the vessel.
27. The process of claim 17 further including the step of releasing
the surface tension of the liquor so that methane gas can bubble
off.
28. The process of claim 17 further including the steps of
reheating the liquor and releasing the surface tension of the
liquor to allow carbon dioxide gas to bubble off.
29. The process of claim 17 including the step of maintaining the
liquor in the vessel at a thermophilic temperature, with the vacuum
created by removing the gaseous products of digestion and the
liquid vapor reducing the pressure at the surface of the liquor to
a level such that the liquid boils at the thermophilic temperature.
Description
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention pertains generally to anaerobic digesters and, more
particularly, to a system and process that significantly extends
the efficiency, control, and applicability of anaerobic digesters
to all of the many and variously different liquefied bio-waste
products over a wide variety of conditions and concentrations.
2. Introduction and Related Art
Designers of wastewater facilities have always been concerned with
energy costs. Historically, however, rather large engineering
"houses" have designed wastewater treatment plants (WWTPs) with
hydraulic loads of millions of gallons per day.
Designing these plants involved large structures, the design
responsibility for each was assigned to a person or group that
specialized in that particular function. A process engineer who was
forced by the dictates of Federal, State, and local agencies to aim
the overall design performance almost entirely with respect to the
effluent requirements of these entities determined the make up of
the process itself. Although energy considerations have in recent
years received some attention little if any serious effort has ever
been directed to the energy savings possible by the integration of
these processes. Evolution not invention predominated the
picture.
As each group perceived a problem in its area of expertise, or if
operational problems developed after construction, enterprising
engineering, inventor, entrepreneur types were brought into the
picture. This approach has resulted in the application of inventive
genius to a lot of fixes, a lot of complexity, and a large increase
in construction, maintenance, and energy costs in WWTPs.
Aerobic And Anaerobic Processes
Domestic wastewater, liquefied bio-waste, commercial and industrial
liquid waste processes have historically used two distinct classes
or systems of bacteria to reduce the biosolids contained therein to
more biochemically safe water and solids that can be used for
fertilizer and a variety of other products. These two bacterial
systems are termed aerobic and anaerobic.
The Aerobic Process
Aerobic processes require the mixing of air or pure oxygen into the
liquor being treated so that aerobies (aerobic bacteria) grow,
attack, and bio-chemically reduce the solids. Aerobic processes are
relatively easy to devise and there are many such systems in use
worldwide.
The drive for higher and higher quality effluents has contributed
to the expansion and proliferation of aerobic processes. However,
there are a number of disadvantages to aerobic processes: they are
in general open processes that have odor problems; they tend to
require large tanks or ponds that require considerable space; and
they consume large quantities of energy in the form of electrical
power. Sixty to seventy percent of the energy required in modern
domestic wastewater treatment plants is directly attributed to
aerobic processes.
The Anaerobic Process
Conversely, anaerobic processes can be net energy producers. They
operate in closed tanks or vessels devoid of oxygen, at an elevated
temperature, are more difficult to control, produce a gas that
contains approximately 64% methane (natural gas), 34% carbon
dioxide, and 2% hydrogen sulfide. It is the methane component of
this raw gas mixture that is valuable for its energy content
(nominally 1000 Btu/ft.sup.3). Gas production rates are a function
of the type and density of the bio-feedstock and general digester
efficiency.
Water To Volatile Solids Ratio
The limiting factor that has prevented all wastewater feedstock
from being treated anaerobicly is the high ratio of water to
bio-solids (volatile solids or VS) contained in the feedstock.
Domestic wastewater typically exhibits as little as 0.01% VS. And
yet, it is normally difficult to maintain anaerobic action below a
minimum threshold of about 3 to 5% VS. Therefore common practice
limits anaerobic digestion to that relatively small part of the
influent that either settles readily or floats to the top of large
primary and secondary sedimentation tanks, thus delegating a very
large portion of the influent to aerobic activated sludge
processes. The invention described herein completely eliminates
this minimum VS requirement so that all biosolid liquor mixtures
may be anaerobicly reduced irrespective of their biosolid (VS)
concentrations.
Previous Anaerobic Limitations
The energy produced by anaerobic systems in the form of methane gas
is a direct function of the quantity of biomass reduced (VSR) in
the process. Therefore, the net positive energy generated is
normally severely limited by the water to VS ratio of the digester
influent, irrespective of the several chemical-thermal-mechanical
factors that determine digester efficiency. And, depending upon the
feedstock there has normally been an operating point at which it
becomes more efficient to delegate a portion of the treated
influent to aerobic processing. This limitation can be overcome to
some extent by the addition of external biosolids such as: food,
animal, agricultural, grass clippings, tree trimming, cardboard,
and other bio-waste products to the anaerobic influent. Therefore,
the ability of this invention to control and maintain the desired
water to VS ratio in the digester eliminates the necessity for, but
not the usefulness of, such considerations.
Anaerobic Temperature
Anaerobic digesters have been operated in a number of temperature
ranges. This invention is applicable to all anaerobic digesters
regardless of temperature. Most common digesters operate in the
mesophilic range of approximately 35.degree. C. or the thermophilic
range (55.degree. C.). The preferred embodiment and the description
of this invention refer to thermophilic operation.
Raw Gas Constituents
In an anaerobic reactor, retort, or vessel operated in the
thermophilic bacterial temperature range of approximately 53 to
58.degree. C. (nominally 55.degree. C.) there is a certain space
above the liquor (hereinafter referred to as the dome, however this
reference does not necessarily limit the shape of the vessel) that
collects the raw gas produced in the reactor by the anaerobic
action. The constituents of this raw gas vary a few percentage
points but generally may be expressed as being 60% methane (CH4),
31% carbon dioxide (CO2), 1% hydrogen sulfide (H2S), and 8% water
vapor (H2O).
The Vacuum Retort
The partial pressures of these gases are a function of the
temperature and pressure in the dome. The quantity of methane,
carbon dioxide, and hydrogen sulfide available to this mixture is
limited by the digester gas production rate. Only the water content
of the liquor, the temperature of the vessel, and the pressure in
the dome however, limit the quantity of the water vapor available.
Since the liquor is generally more than 95% water the quantity of
water vapor available may be considered infinite within the
confines of this discussion. And, since the temperature of the
vessel is set by the anaerobic requirement the surface of the
liquor may be considered constant at 55.degree. C. However, the
total pressure and to some extent the temperature in the gaseous
space of the dome above the liquor may be varied widely and in
itself will have virtually no effect upon the temperature or
operation of the digestion process. Decreasing the pressure will
increase the partial pressure of water vapor increasing the ratio
of water vapor to gas. It is a major action and purpose of this
invention to decrease the absolute dome pressure (creating a
vacuum); thus increasing the percentage of water vapor; drawing off
this water vapor; to result in the lowering of the water to VS
ratio in the vessel, at a rate and to an extent that maximizes
methane production and VSR.
The Retort Process
At the temperature of 55.degree. C. water boils under a vacuum of
12 psi relative to standard conditions. In the range between
atmospheric pressure (14.7 psia) and 12 psi vacuum (3 psia) the
water vapor available to be drawn off by the process increases in a
linear fashion and heat is drawn from the process. At the boiling
point however, the rapid boiling of the water impedes further
reduction in pressure. At this low pressure the rate of boiling is
a function of the heat of vaporization resulting in a sharp rise in
the heat drawn from the system. There is a sharp rise in additional
heat that must be applied to the vessel influent in order to
maintain thermophilic temperature. This invention provides for the
condensation and heat exchange of this primarily water vapor or
steam-gas mixture flow to the influent feedstock dramatically
reducing the requirement for make up heat to the system. The vacuum
created in the retort dome is created and controlled by condensing
action and the pumping or pulling off by gravity of this condensate
gas mixture. The pumping of this predominantly water mixture by gas
diffusion liquid pumping or elevated water column condensation are
the preferred embodiments of this invention. Embodiments may be
performed by special vortex, centrifugal, or other technology. This
invention additionally makes use of the facts: that biosolid
feedstock at higher operating temperatures demonstrate improved
solids separation characteristics; that hot water, vapor, or steam
introduced above the liquor level is more readily drawn off into
the condenser heat exchange unit; and that heated feedstock
introduced to the dome above the scum layer helps to break up this
layer, reduces foaming, and makes use of the additional surface
area of this rough and uneven surface to enhance vaporization in
the dome.
Raw Gas Mixing
Typically the Raw Gas generated in wastewater, solid waste and/or
landfill processes is recycled in order to provide mixing of the
liquor in the digestion process. The use of raw gas in mixing is an
aid to bacterial growths that break down the bio-solids in the
anaerobic digestion process. Raw gas is recycled in the digestion
process by one of various methods. The process described herein
applies to all gas mixing methods.
Acid And Fermentation Phases
Anaerobic digestion is basically a two-phase process. It is the
combined action of two forms of bacteria that live together in the
same environment and are commonly referred to as the "acid formers"
and "methane fermenters". It has been found advantageous to
separate or transition these phases to some degree as an aid to
keeping a balance between these two bacteria. Acid formers are
abundant in raw sewage. Methane fermenters are not nearly so
prevalent and require a pH of about 6.6 to 7.6 to produce. A
digester is sensitive to too much food, it may easily become too
acidic and "go sour" and fail to produce the desired innocuous
dewaterable sludge and valuable methane (CH4) if the acid phase is
allowed to predominate. The method described herein separates the
components of the mixing gas, controls the volumetric gas flow, and
stabilizes the above condition over a much broader range of feed
conditions by regulating the content of the mixing gas to the
various phases.
Gas Constituents In Mixing
The carbon dioxide content forms carbonic acid (H2CO3), which when
returned to the digester as a component of the mixing gas moves the
balance in the direction of the acid formers and can be used to
enhance their activity in the acid phase. However the methane
component is a valuable constituent both as a gas that may be drawn
off the system for energy and as a mixing gas. Many believe that
its presence is a further aid to the health of methane fermenters.
Natural gas, which contains a high percentage of methane, is
sometimes used to restart sour (acidic) digesters. As a mixing gas
methane slows the acid forming phase and enhances the fermenting
phase. The water vapor and hydrogen sulfide gas that form
hydrosulfuric acid a destructively active acid component that makes
itself a general nuisance by condensing out in lines, corroding
expensive equipment, and the instrumentation used to monitor and
control gas flow are entirely removed from the process.
Separating Gas Constituents
Several methods for separating (scrubbing) methane from the other
raw gas constituents have been developed. The application of these
processes to wastewater treatment plants has not proven practical
and/or economically feasible. The method and process described
herein not only provides clean methane gas that is dry cold and
dense and does it at atmospheric pressure but also overcomes all of
the drawbacks connected with other processes and enhances the
operational efficiency of the plant operation.
Thermophilic Operation
The Thermophilic Digestion Process is not new. However, like the
various gas scrubbing techniques, it has seldom previously been
considered an economically viable solution to the treatment of
biosolids in a full sized digester. Until the advent of this
invention there has never been available an economical heat source
capable of maintaining the additional 20.degree. C. required for
thermophilic operation. Thermophilic digestion is three (plus)
times as fast as mesophilic digestion. For instance, this process
can reduce the same amount of volatile solids in 10 days that a
mesophilic digester will reduce in 32 days. It follows that there
is three (plus) times the production of methane gas. By providing
the heat required to raise and maintain the operating temperature
at the thermophilic range the "basic" digester operating efficiency
has increased by a factor of three.
Raw Gas Operation Limitations
Utilization of the methane energy component of this raw gas has
been hampered by the presence of the other by-product components of
anaerobic digestion. The most insidiously harmful of these
components with respect to repair, maintenance and replacement of
equipment in the mixing and/or gas to energy systems (i.e. pumps,
blowers, compressors, boiler tubes, cylinders, etc.) is the
condensing water vapor and the dilute hydrosulfuric acid (H.sub.2
SO.sub.4 +H.sub.2 O) product of the hydrogen sulfide and water
constituents. Operations personnel soon become discouraged by the
unreliability, high maintenance costs, and the low time between
failures associated with these equipments. So much so in fact that
many plants have abandoned the use of raw gas altogether in favor
of natural gas (domestic or pipeline), opting to flare-off the raw
digester gas and its harmful components to the atmosphere. The
invention herein described greatly increases the traditional mean
time between failure (mtbf) of all of these equipments.
In addition, the volumetric inefficiency of using a gas that is 40%
inert in gas-engines used for pumping or generating systems
requires much larger and more expensive engines than the service
requirement would otherwise dictate, accompanied by a corresponding
reduction in operating efficiency. Additionally passing this carbon
dioxide through the combustion process increases the "green house"
effect upon the atmosphere. The invention herein described
eliminates all of these drawbacks and limitations providing a
positive environmental impact.
Sludge Thickening
Secondary sludge (i.e. sludge from the secondary sedimentation
basins and the aerobic treatment processes) tends to be thinner
than primary sludge (i.e. sludge from the primary sedimentation
basins). In order to handle secondary sludge effectively, whether
it is used in direct land application or cycled through the
digester, it is customary to thicken this sludge. This requires
rather elaborate and expensive apparatus with certain chemicals
such as polymers to aid the process. By cycling all sludge through
the retort digester the requirement for a separate sludge
thickening process is eliminated. Thickening of the sludge in the
digester takes place automatically and continually as the water is
removed from the vacuum retort digester.
Pasteurization
Most recently the United States Environmental Protection Agency in
the Code of Federal Regulations 40 Section No. 503 has mandated
that in order for sludge to be classified as Class A for
unrestricted use or access land application it must be pasteurized
to below harmful pathogens levels. Subjecting sludge to
thermophilic temperatures for as little as one quarter hour
provides this pasteurization. Systems of this type have been in use
in Europe for a number of years and are becoming more prevalent in
the United States. The equipment involved in such a process is
expensive, and only augments digestion process meanwhile adding
complexity to the treatment process. The method and process
contained herein subjects all sludge to pasteurization, completely
eliminating the need for a separate process while providing a
positive environmental impact.
Effluent Ph
When applied to retrofit digesters or installed as a partial
treatment process in an existing plant in which the distilled
process water is to be added to the plant effluent only partial
de-carbonization need take place in the system. The remaining
carbonization buffers effluent water. Typically the effluent of
wastewater treatment plants tends to run alkaline (i.e., pH in
excess of 7.0). In a large number of plants with certain types of
industrial influent the pH range becomes excessive. Equipment is
being installed in WWTPs today solely for the injection of bottled
CO2 gas into the effluent prior to chlorination or other
disinfection process. Carbon dioxide provides superior process
control by virtue of its self-buffering characteristics. It is
being used to replace older systems that use hydrochloric acid,
sulfuric acid or acetic acid. The method and process contained
herein completely eliminates the need for either of these systems.
The carbon dioxide and hydrogen sulfide removed from the raw
digester gas is dissolved in the water that is inserted into the
secondary effluent stream. In addition, maintaining this buffer
reduces the amount of chlorine required for disinfection and
reduces the discharge of carbon dioxide to the atmosphere.
OBJECTS AND SUMMARY OF THE INVENTION
It is in general an object of the invention to provide a new and
improved anaerobic digestion system and process.
Another object of the invention is to provide an anaerobic system
and process of the above character which provide 100% energy
independence, zero waste products, zero environmental pollution,
potable distilled water production, and a pasteurized fertilizer,
sludge or sludge cake product, in a closed bio-thermodynamic
systems.
These and other objects are achieved in accordance with the
invention by: (a) Concentrating the liquor in the digester and
boiling off water in excess of the amount required for optimizing
the digestion process, by creating and controlling a vacuum in the
enclosed space above the liquor; (b) Producing clean, dry, pure
methane gas from the raw gas produced in the digester, as disclosed
in U.S. Pat. No. 6,291,232, and utilizing that gas to power and
control the process and as a saleable pipeline quality gas and/or
electrical energy producing product; (c) Producing pure carbon
dioxide gas, by de-carbonating the effluent water, and utilizing
that gas to control the digester operation and as a saleable
by-product of the process; (d) Controlling the pH of the digester
to increase the digestive action without the use of chemical
additives (Steiner 5,630,492), by selectively reintroducing the
separated components of the raw gas into the several phases or
stages of the digester as an injection additive to the mixing
liquor or as the mixing gas itself; (e) Providing a biologically
and chemically pure liquid or potable distilled water effluent by
condensing out the water vapor produced in paragraph (a) above; and
stripping out all the dissolved gas constituents of that
condensate; (f) Providing a system and process for 100% digestible
volatile solids reduction (VSR), that provides extreme flexibility
and control of solids retention time (SRT), and insures the
production of completely pasteurized biosolids by eliminating the
possibility of short-circuiting within the digester from effecting
pasteurization; (g) Providing a vacuum drying system for post
digester sludge drying and/or sludge cake production, by extending
and utilizing the vacuum created in the digester dome to a drying
oven; and (h) Producing the useful, saleable chemical by-product
sodium sulfate (Epson Salts, and an ingredient in detergents, ink
and other products), as a first step precipitant during the
reactive neutralization of the chilling water supply by the
reduction of hydrogen sulfide with sodium hydroxide.
The invention may be used as a standalone liquid waste (wastewater)
plant or as a progressive retrofit or addition to an existing
plant. This invention is applicable to and improves the cost
effectiveness of wastewater treatment plants with flows as low as a
few thousand gallons per day to plants with flows of several
hundred million gallons per day and with biomass concentrations as
low as 0.005% VS (volatile solids) concentrations. The invention is
applicable to: all currently known digester systems, including but
not limited to single and multi-phase, multi-staged, temperature
phased (U.S. Pat. No. 5,525,228), acid phased, mesophilic,
thermophilic, suspended growth, up-flow, down-flow, granular,
enhanced granular, fluidized bed, attached growth, and the various
combinations of the same; including but not limited to those
digesters that employ filter media whether fully or partially
packed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of one embodiment of an anaerobic
digestion system incorporating the invention as might, for example,
be utilized in a domestic wastewater plant that has retrofitted its
digester process or is splitting its flow between aerobic and
anaerobic processes.
FIG. 2 is a schematic diagram of another embodiment of an anaerobic
digestion system incorporating the invention which utilizes a water
to volatile solids ratio that results in operation in a higher
energy mode and eliminates the need for a major heat exchanger.
FIG. 3 is a schematic diagram of one embodiment of another
anaerobic digestion system incorporating the invention as might,
for example, be utilized in a self-contained, fully anaerobic
wastewater treatment process in which the system process water is
used as the dissolution medium.
FIG. 4 is a more detailed schematic diagram of an embodiment of a
single stage multi-phase anaerobic digester unit with pH control
which is particularly suitable for use in the embodiments of FIGS.
1, 2, 3, 6, and 7.
FIG. 5 is a schematic diagram of one embodiment of a multi-stage
(series connected), multi-phase anaerobic digester bank with pH
control which is particularly suitable for use in the embodiments
of FIGS. 1, 2, 3, 6, and 7.
FIG. 6 is a schematic diagram of another embodiment of an anaerobic
digestion system according to the invention, which is suitable for
use as a stand-alone process system with full pH and sludge
thickening control, and process optimization.
FIG. 7 is a hydraulic profile diagram of and similar to the
embodiment of FIG. 6, utilizing the force of gravity to produce a
vacuum in the retort dome. For convenience and clarity of
illustration, details such as valves, indicators, sensors, pumps,
and computers, are omitted from the drawings except as where
necessary to clarify the action of the system and process.
Likewise, commonly used pretreatment and preprocessing equipment
such as mixers, screens, and grinders is not shown.
DETAILED DESCRIPTION
In the embodiment of FIG. 1, raw sludge removed from the primary
and secondary sedimentation tanks is directed to the process at 1
where it is introduced to heat exchanger 2, raising its
temperature. The raw sludge is additionally passed through vapor
condenser and heat exchanger 3 and 4 to bring its temperature to
above the thermophilic range before being passed through line 5 to
the digester 6, insuring that any possible short circuiting in the
digester will not prevent pasteurization. After digestion, the
thickened sludge 7 returns to heat exchanger 2 before passing to
sludge disposal line 8. A hot water or steam boiler 9 together with
water jacket heat from engine generator 10 forms a heat loop 11
supplying heat to exchanger 4 and digester 6. Gas and water vapor
exit the dome of the digester 12. The vapor of the mixture is
condensed in 3 and passes along with the gas to pre-chiller 13
where it is further cooled and passes to gas diffuser pump 14. The
combined action of condensing 3, cooling 13, and pumping 14 results
in a vacuum in dome 12 that acts to increase the water vapor
percentage of the water gas mix allowing for increased pumping at
14 until the desired water removal rate from the digester 6 has
been reached. The water-gas mixture leaving pump 14 is
pre-separated at 15, and water 16 is directed to the plant effluent
line 17, while the gasses are directed to main gas diffuser and
pump 18 with a bleed off to methane and carbon dioxide controller
19.
Sufficient water flow 20 is slipstreamed off the plant effluent
line 17 to insure complete dissolution of carbon dioxide and
hydrogen sulfide at a temperature close to freezing in diffuser 18.
Water 20 passes through system chiller 22 to diffuser pump 18 where
carbon dioxide and hydrogen sulfide are completely dissolved in
solution leaving methane to be separated in 23. The cold water
containing the dissolved gasses pass through pre-chiller 13, heat
exchanger 21, and refrigeration condenser 24 absorbing heat and
pre-cooling these items to reduce the load on compressor 25.
Pure methane from separator 23 is directed to methane distribution
26, is used to power engine generator 10, boiler 9, as digester pH
control 28 through controller 19, and may be optionally sold, as
indicated at 27. Electrical power from engine generator 10 is
distributed at 29, powers compressor 25, and plant electrical loads
30.
In the embodiment of FIG. 2, raw sludge removed from the primary
and secondary sedimentation tanks is directed to the process at 1
where it is introduced to heat exchanger 2, raising its
temperature. The raw sludge is additionally passed through vapor
condenser and heat exchanger 3 and 4 to bring its temperature to
above the thermophilic range before being passed 5 to the digester
6, insuring that any possible short circuiting in the digester will
not prevent pasteurization. After digestion, the thickened sludge 7
returns to heat exchanger 2 before passing to sludge disposal line
8. A hot water or steam boiler 9 together with water jacket heat
from engine generator 10 forms a heat loop 11 supplying heat to
exchanger 4 and digester 6. Gas and water vapor exit the dome of
the digester 12. The vapor of the mixture is condensed in 3 and
passes along with the gas to pre-chiller 13 where it is further
cooled and passes to gas diffuser pump 14. The combined actions of
condensing 3, cooling 13, and pumping 14 result in a vacuum in dome
12 that acts to increase the water vapor percentage of the water
gas mix allowing for increased pumping of this distilled water and
gas mixture at 14 until the desired water removal rate from the
digester 6 has been reached. The water gas mixture leaving pump 14
is pre-separated at 15, water 16 is directed to the plant effluent
17, while the gases are directed to main gas diffuser and pump 18
with a bleed off to methane and carbon dioxide controller 19.
Sufficient water flow 20 is slipstreamed off the plant effluent
line 17 to insure complete dissolution of carbon dioxide and
hydrogen sulfide at a temperature close to freezing in diffuser 18.
Water 20 passes through system chiller 22 to diffuser pump 18 where
carbon dioxide and hydrogen sulfide are completely dissolved in
solution leaving methane to be separated in 23. The cold water
containing the dissolved gasses pass through pre-chiller 13 is
joined with distilled water 16 added to plant effluent 17. Upstream
Water 17 is passed through refrigeration condenser 24 absorbing
heat and cooling the condenser as dictated by the load on
compressor 25.
Pure methane from separator 23 is directed to methane distribution
26, is used to power engine generator 10, boiler 9, as digester pH
control 28 through controller 19, and may be optionally sold 27.
Electrical power from engine generator 10 is distributed at 29,
powers compressor 25, and plant electrical loads 30.
In the embodiment of FIG. 3, liquefied biomass feedstock is
directed to the process at 1 where it is introduced to heat
exchanger 2, raising its temperature. The feedstock is additionally
passed through condenser and heat exchanger 3 and 4 to bring its
temperature to above the thermophilic range before being passed 5
to the digester 6, insuring that any possible short circuiting in
the digester will not prevent pasteurization. After digestion, the
thickened sludge 7 returns to heat exchanger 2 before passing to
sludge disposal line 8. A hot water or steam boiler 9 together with
water jacket heat from engine generator 10 forms a heat loop 11
supplying heat to exchanger 4 and digester 6. Gas and water vapor
exit the dome of the digester 12. The vapor of the mixture is
condensed in 3 and passes along with the gas to pre-chiller 13
where it is further cooled and passes to gas diffuser pump 14. The
combined actions of condensing 3, cooling 13, and pumping 14 result
in a vacuum in dome 12 that acts to increase the water vapor
percentage of the water gas mix allowing for increased pumping of
this distilled water and gas mixture at 14 until the desired water
removal rate from the digester 6 has been reached. The water gas
mixture leaving pump 14 is pre-separated at 15, water 16 is mixed
with gas-water mixture 32 and directed to gas removal 80 and thence
to storage 31, while the gases are directed to main gas diffuser
and pump 18 with a bleed off to methane and carbon dioxide
controller 19.
Sufficient water flow 20 is directed from storage tank 31 to insure
complete solution of carbon dioxide and hydrogen sulfide at a
temperature close to freezing in diffuser 18. Water 20 passes
through system chiller 22 to diffuser pump 18 where carbon dioxide
and hydrogen sulfide are completely dissolved in solution leaving
methane to be separated in 23. The cold water containing the
dissolved gasses pass through pre-chiller 13, line 32 and is joined
with distilled water 16, stripped of carbon dioxide and hydrogen
sulfide 17 and added to storage 31. Storage water 31 is cycled
through refrigeration condenser 24 absorbing heat and cooling the
condenser as dictated by the load on compressor 25.
Pure methane from separator 23 is directed to methane distribution
26, is used to power engine generator 10, boiler 9, as digester pH
control 28 through controller 19, and may be optionally sold 27.
Electrical power from engine generator 10 is distributed at 29,
powers compressor 25, and plant electrical loads 30.
In FIG. 4, a preferred embodiments of invention a multi-phase
digester is depicted schematically. Grid separation and partially
packed floating media are depicted together with the liquid-gas
mixing and pH control system. Pre-heated biomass feedstock 5 is fed
to digester dome 12, where its vapor content along with the gas
generated in the digester phases 33, 34, 35, and 36 are removed
under vacuum at line 37 and a concentrated digested sludge is
removed at line 7. Grids 39 and 40 separate the floating media 41,
42, and 43. Digester phases 33, 34, 35, and 36 are maintained at
thermophilic temperature (55.degree. C.) by heat loop 141.
Liquid-gas mixing maintained by gas diffusion pumps 45, 46, and 47
together with substrate return pump 48. Automatic control valves 49
through 54 regulate the gas or gas mixture from methane supply 55
and/or carbon dioxide supply 56 as required to maintain the optimum
pH in each phase.
FIG. 5 illustrates a multi-stage, multi-phase digester with feed
and pH control. Liquefied biomass feedstock (domestic and/or other)
is applied 4 at 5 to the first stage digester 92, through line 93
to second stage digester 94 and finally to third stage digester 96
through line 95. Digested sludge is directed from the final stage
by way of line 7. Gas diffusion pumps 45, 46, and 47 circulate the
liquid-gas mixture in each stage 92, 94, and 96. Automatic control
valves 49 through 54 control the pH in each stage and/or phase by
adjusting the flow of methane 55 and/or carbon dioxide 56 to the
diffusion pumps 45, 46, and 47. Automatic control valves 57, 58,
and 59 isolate the stages and control the relative dewatering rates
of each stage by controlling the vacuum in domes 63, 64, and 65.
Circulating pumps 60, 61, and 62 re-circulate substrate in each
stage and pump 48 re-circulates substrate from the last stage 96 to
digester 92 to insure favorable inoculums.
FIGS. 6 and 7 illustrate two similar systems which employ different
methods of creating a vacuum in the dome of the digester. In the
embodiment of FIG. 6, the vacuum created in the dome is a function
of pumping as in the embodiments of FIGS. 1, 2, and 3. In the
embodiment of FIG. 7, the vacuum is created in the dome by
condensing the vapor at a distance above the dome equal to
approximately one atmosphere of water column. The choice between
the two can be made on the basis of factors such as capital and
operating costs that may vary case to case.
In the embodiment of FIG. 6, liquefied biomass feedstock 1
(domestic and/or other) is progressively fed and heated through
energy recovery heat exchanger 2, vacuum condenser 3, and hot
water/steam heat exchanger 4, to enter the digester(s) dome 12,
where the liquid/vapor fraction that is surplus to digester
operation and the gasses produced by digestion are drawn off by the
vacuum created from the combined action of condenser 3, pre-chiller
heat exchanger 13, chiller evaporator 22, and gas diffusion pump of
methane separator 23. A measured amount of sodium hydroxide
solution 38 is injected into the vapor-gas vacuum line at the dome
outlet 12 in order to neutralize the very active hydrosulfuric acid
formed by the hydrogen sulfide gas and water. This chemical
reaction produces a useful by-product sodium sulfate (mirabilite)
Na2SO4+10H2O that crystallizes at lower temperatures and is removed
as a residue from methane separator 23. The distilled water and
dissolved carbon dioxide are warmed as they pass through the
condenser 24 of refrigeration compressor 25, pre-chiller 13. The
bulk of the carbon dioxide gas is stripped from the water in
separator 80, part of which goes to distribution in the system 56
and the balance is pumped to storage and sale via line 66. Water
from separator 80 passes through energy recovery heat exchanger 2
and out of the system through line 68.
Boiling off the influent water 5 at or near the same rate which it
arrives to the process 1 maintains a constant liquid level in the
digester dome 12 and allows extended SRTs and maximizes methane
production. Methane from distribution element 26 is used to control
digester pH, power the boiler 9 and the electrical engine generator
10, and may be sold or used for other purposes via output line 27.
Electrical power from engine generator 10 is directed to electrical
distribution 29, thence to compressor 25 and other plant loads.
Boiler 9 and heat jackets on engine generator 10 provide heat for
the exchanger 4, digester 6, and vacuum sludge thickener oven 67.
Sludge pump 69 supplies sludge-to-sludge cake vacuum oven 67 and
vacuum line 44 carries away its vapor and gasses.
Digester mixing and control in this embodiment are similar to the
mixing and control in the embodiments of FIGS. 4 and 5.
In the embodiment of FIG. 7, liquefied biomass feedstock 1 is
pumped 70 into the base of shell and tube vertical heat exchange
tower 71 where it passes through heat exchange sections 72, 73, 74,
and 75 in which it is progressively heated until it is finally
exited, through line 5 at an elevated temperature of from 55 to
100.degree. C. Effluent in line 5 is directed into the vacuum dome
12 of one of several typical digester configurations 6 where it
deposits its biomass solids to the digester 6. The bulk of effluent
(nominally 95 to 99% water) either exits the dome directly in the
form of water vapor or is added to the digester content as make up
for digested sludge draw off 7. Water vapor and defused gases
produced by digestion rise in vacuum line 37 and enter the second
vertical heat exchange tower 76 where the vapor is cooled,
condensed to water and progressively chilled in heat exchange
sections 77 and 78. The condensing point in exchange tower 76 is
maintained at approximately the distance of one atmosphere water
column above dome 12 and the pipe velocities are maintained at
above diffused gas bubbling point in order to keep the vacuum
created by this arrangement from being broken.
Due to design flow rates and vapor to gas ratios this vacuum
condition can be hard to control, in which case diffusion pumping
at can be installed between evaporator 22 and separator 23, if
desired.
Effluent from tower 76 enters exchange unit 73 of tower 71 where it
is further step cooled before entering pre-chiller heat exchanger
13 and thence to chiller evaporator 22, where it emerges through
line 88 at a temperature of 0 to 2.degree. C. for separating the
methane component in separater 23. Water and dissolved carbon
dioxide flow from separator 23 to condenser 24 where heat removed
by compressor 25 is replaced and then increased in pre-chiller 13
before being directed to heat-exchanger 79, where it is heated to
30.degree. C. for carbon dioxide separation in separater 80. Water
level in separator 80 is maintained by discharge pump 81 and passes
through unit 73 of tower 71 to final effluent line 68.
Control valve 89 cycles distilled water back to pre-chiller heat
exchanger when higher VS loadings produce more carbon dioxide than
water from influent stream can dissolve. Carbon dioxide is removed
for pH control and sale through 83. Methane is taken off at 55 to
power the system, provide pH control in the digester and for sale
or further utilization. Sodium sulfate precipitate is removed at 82
and sand/grit that may have entered the tower 71 is removed at 83.
Pump 86 circulates heat generated in vapor condenser 77 to heat
exchanger 79 through heating loop 85. Refrigeration compressor 85
removes heat from condensed vapor at evaporator 78 and supplies
that heat to tower 71 at condenser exchanger 74 through
refrigeration loop 87. Steam from boiler (FIG. 6) transmitted
through loop 11 provides final heat to digester feedstock 1 at
exchanger 75 of tower 71, and regulates the vaporization rate in
dome 12 to meet digestion operational requirements.
The invention has a number of important features and advantages. It
has the ability to process wastewaters in a simple, clean, closed
system, and it can process an almost limitless variety of bio-waste
substances with wide variations in biochemical oxygen demand. It
produces a potable water effluent, clean commercially useful
by-products, and zero environmental emissions, which means it has a
very positive ecological impact. It can be completely self powered
and provide energy back to the community or industry that supports
or owns it. It will change forever the world's concept of the
wastewater industry and is a major step forward in our society.
It is apparent from the foregoing that a new and improved anaerobic
digester and process have been provided. The embodiments disclosed
herein are only exemplary, and as will be readily apparent to those
skilled in the art, differences such as variations in flow path,
location of heat exchange units and other auxiliary process
equipment will occur in order to meet the requirements of each
individual installation such as the type and density of feedstock,
feed rate, the final moisture content desired in the effluent
sludge, and the relative economic importance of overall digestion
efficiency and system byproducts.
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